Natural gas, found in deposits in the earth, is an abundant energy resource. For example, natural gas commonly serves as a fuel for heating, cooking, and power generation, among other things. The process of obtaining natural gas from an earth formation typically includes drilling a well into the formation. Many wells that provide natural gas are remote from locations with a demand for the consumption of the natural gas, creating the need to transport the gas large distances from the wellhead to commercial destinations. Because the volume of an amount of gas is so much greater than the volume of the same number of gas molecules in a liquefied state, the process of transporting natural gas often includes chilling and/or pressurizing the natural gas in order to liquefy it. However, liquefaction contributes to the final cost of the natural gas.
Thus, there has been interest in developing technologies for converting natural gas to more readily transportable liquid fuels, i.e. to fuels that are liquid at standard temperatures and pressures. One method for converting natural gas to liquid fuels involves two sequential chemical transformations. In the first transformation, natural gas or methane, the major chemical component of natural gas, is reacted with oxygen, or steam, or a combination of both, to form synthesis gas, which is a combination of carbon monoxide and hydrogen. In the second transformation, known as Fischer-Tropsch synthesis, carbon monoxide is reacted with hydrogen to form organic molecules containing carbon and hydrogen. Those molecules containing only carbon and hydrogen are known as hydrocarbons. Those molecules containing oxygen in addition to carbon and hydrogen are known as oxygenates. Hydrocarbons having carbons linked in a straight chain are known as aliphatic hydrocarbons and are particularly desirable as the basis of synthetic diesel fuel.
The Fischer-Tropsch process is commonly facilitated by a catalyst. Catalysts desirably have the function of increasing the rate of a reaction without being consumed by the reaction. Common catalysts for use in the Fischer-Tropsch process contain at least one metal from Groups 8, 9, or 10 of the Periodic Table (in the new IUPAC notation, which is used throughout the present specification). The molecules react to form hydrocarbons while confined on the surface of the catalyst. The hydrocarbon products then desorb from the catalyst and can be collected. H. Schulz (Applied Catalysis A: General 1999, 186, p 3) gives an overview of trends in Fischer-Tropsch catalysis.
The catalyst may be contacted with synthesis gas in a variety of reaction zones that may include one or more reactors. Common reactors include packed bed (also termed fixed bed) reactors and slurry bed reactors. Originally, the Fischer-Tropsch synthesis was carried out in packed bed reactors. These reactors have several drawbacks, such as temperature control, that can be overcome by gas-agitated slurry reactors or slurry bubble column reactors. Gas-agitated multiphase reactors sometimes called “slurry reactors” or “slurry bubble column reactors,” operate by suspending catalytic particles in a liquid phase and feeding a gas phase comprising reactants into the bottom of the reactor through a gas distributor, which produces small gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they are typically converted to gaseous and liquid products. The gaseous products formed enter the gas bubbles and are collected at the top of the reactor. Liquid products are recovered from the suspending liquid by using different techniques like filtration, settling, hydrocyclones, magnetic techniques, etc. Some of the principal advantages of gas-agitated multiphase reactors or slurry bubble column reactors (SBCRS) for the Fischer-Tropsch synthesis, which is exothermic, are the very high heat transfer rates and the ability to remove and add catalyst online. Sie and Krishna (Applied Catalysis A: General 1999, 186, p. 55) give a history of the development of various Fischer Tropsch reactors.
Typically the Fischer-Tropsch product stream contains gas, liquid, and wax hydrocarbon products having a range of number of carbon atoms from 1 to 100 or more, and thus having a range of molecular weights. It is highly desirable to maximize the production of high-value liquid hydrocarbons, such as hydrocarbons with 5 to 20 carbon atoms per hydrocarbon chain (C5–C20 hydrocarbons). Throughout the specification Cn+ denotes hydrocarbons containing at least n hydrocarbons, that is n or more hydrocarbons. These include, for example, wide range naphtha fractions, such as fractions containing C5–C12 hydrocarbons, useful for processing to gasoline, and gasoil fractions, such as fractions containing C13–C20 hydrocarbons, useful for processing to diesel oil. Heavier hydrocarbons in the wax range (e.g. C20+) can undergo hydroprocessing treatments to lower their molecular weights to obtain a desirable range of carbon chain length as well as to increase the degree of skeletal isomerization which might be necessary for improving properties of the gasoline and diesel fuels. Lower molecular weight products, such as C1–C4 hydrocarbons, tend to be gaseous at room temperature and are not typically among the desired products.
Typically, in the Fischer-Tropsch synthesis, the distribution of weights that is observed such as for C5+ hydrocarbons, can be described by likening the Fischer-Tropsch reaction to a polymerization reaction with an Anderson-Shultz-Flory chain growth probability (α) that is independent of the number of carbon atoms in the lengthening molecule. Thus, a range of hydrocarbons from C1 to C21+ may be formed, with a selectivity to liquids that depends on the production of gaseous hydrocarbons, as well as on α. In particular, the selectivity to liquids in the non-gaseous product is typically characterized by α. α is typically interpreted as the ratio of the mole fraction of Cn+1 product to the mole fraction of Cn product. A value of α of at least 0.72 is preferred for producing high carbon-length hydrocarbons, such as those of diesel fractions and higher molecular weights.
One factor limiting the rate of reaction and selectivity toward the desired higher weight hydrocarbons is the hydrogen (H2) and carbon monoxide (CO) concentrations in the liquid phase. The concentrations of H2 and CO in the liquid phase are dependent on, and may be limited by, the mass transfer coefficient (kLa) across the bubbles in the slurry and the solubility of the reactants into the liquid portion of the slurry. Both the mass transfer rate and the solubility are dependent on the molecular weight of the slurry and decrease as the molecular weight of the slurry increases. Therefore, although it may be desirable to produce higher molecular weight hydrocarbons, these hydrocarbons decrease the productivity of the reactor.
Thus, there remains a need in the art for methods and apparatus for reducing the molecular weight of the slurry in order to improve reaction rate, conversion, and reactor production. Therefore, the embodiments of the present invention are directed to methods and apparatus for reducing the molecular weight of a slurry that seek to overcome the limitations of the prior art.